Abstract

Background

Central neurotensin (NT) administration results in a naloxone-insensitive antinociceptive
response in animal models of acute and persistent pain. Both NTS1 and NTS2 receptors
were shown to be required for different aspects of NT-induced analgesia. We recently
demonstrated that NTS2 receptors were extensively associated with ascending nociceptive
pathways, both at the level of the dorsal root ganglia and of the spinal dorsal horn.
Then, we found that spinally administered NTS2-selective agonists induced dose-dependent
antinociceptive responses in the acute tail-flick test. In the present study, we therefore
investigated whether activation of spinal NTS2 receptors suppressed the persistent
inflammatory pain symptoms observed after intraplantar injection of formalin.

Results

We first demonstrated that spinally administered NT and NT69L agonists, which bind
to both NTS1 and NTS2 receptors, significantly reduced pain-evoked responses during
the inflammatory phase of the formalin test. Accordingly, pretreatment with the NTS2-selective
analogs JMV-431 and levocabastine was effective in inhibiting the aversive behaviors
induced by formalin. With resolution at the single-cell level, we also found that
activation of spinal NTS2 receptors reduced formalin-induced c-fos expression in dorsal horn neurons. However, our results also suggest that NTS2-selective
agonists and NTS1/NTS2 mixed compounds differently modulated the early (21–39 min)
and late (40–60 min) tonic phase 2 and recruited endogenous pain inhibitory mechanisms
integrated at different levels of the central nervous system. Indeed, while non-selective
drugs suppressed pain-related behaviors activity in both part of phase 2, intrathecal
injection of NTS2-selective agonists was only efficient in reducing pain during the
late phase 2. Furthermore, assessment of the stereotypic pain behaviors of lifting,
shaking, licking and biting to formalin also revealed that unlike non-discriminative
NTS1/NTS2 analogs reversing all nociceptive endpoint behaviors, pure NTS2 agonists
specifically inhibited paw lifting, supporting a role of NTS2 in spinal modulation
of persistent nociception.

Conclusion

The present study provides the first demonstration that activation of NTS2 receptors
produces analgesia in the persistent inflammatory pain model of formalin. The dichotomy
between these two classes of compounds also indicates that both NTS1 and NTS2 receptors
are involved in tonic pain inhibition and implies that these two NT receptors modulate
the pain-induced behavioral responses by acting on distinct spinal and/or supraspinal
neural circuits. In conclusion, development of NT agonists targeting both NTS1 and
NTS2 receptors could be useful for chronic pain management.